Three-dimensional sizes and shapes of pion emission in heavy-ion collisions

TL;DR

This work performs a three-dimensional femtoscopic analysis of pion emission in Au+Au collisions at $\\sqrt{s_{\\rm NN}}=200$ GeV using EPOS3 Monte Carlo simulations, focusing on Lévy-stable source shapes and a core-halo decomposition. The authors reconstruct the 3D pair-source distribution in LCMS, fit a 1D Lévy-stable core-core component along out/side/long projections to extract the Lévy exponent $\alpha$, Lévy-scale radii $R_{\rm out,side,long}$, and the correlation strength $\lambda$ across multiple $m_T$ and centrality bins. They compare their results to recent PHENIX measurements, finding that EPOS3 reproduces the qualitative trends of $\alpha$ and $R_{\rm out/side/long}$ and the scaled $\lambda/\lambda_{\max}$, but exhibits quantitative deviations in central collisions, particularly for $\alpha$ and unscaled $\lambda$. The findings suggest that hydrodynamics-based transport models with hadronic dynamics capture key femtoscopy observables but may require additional physics, such as Coulomb interactions or in-medium hadron modifications, to fully describe central collisions.

Abstract

In the era of precision measurements in high-energy heavy-ion physics, there is an increasing expectation towards phenomenological and theoretical studies to provide a better description of data. In recent years, multiple experiments have confirmed through two-pion Bose-Einstein correlation measurements that the shape of the two-pion pair source can be well described by Levy-stable distributions. However, direct comparisons of new phenomenological results with the data are still needed to understand the underlying phenomena and learn more about the nature of pion emission. In this paper, we present a three-dimensional analysis of the two-pion source in Monte-Carlo simulations of Au+Au collisions at 200 GeV per nucleon collision energy, and discuss a detailed comparison with the most recent centrality-dependent measurements from the PHENIX Collaboration.

Figures (8)

• Figure 1: An example $D(\boldsymbol{\rho})$ distribution for 30--40% centrality and $k_T$ within $[0.425,0.475]$ GeV/$c$, projected in the out direction. The kinematical track ($\eta$, $p_T$) and pair ($k_T$, $Q_{\rm LCMS}$) selection criteria are also shown. Above the horizontal axes typical parent particles---producing the vast, $>99\%$ majority of pions---in the given $\rho$ range are indicated.
• Figure 2: An example fit to projected $D(\boldsymbol{\rho})$ distributions for 30--40% centrality and $k_T$ within $[0.425,0.475]$ GeV/$c$. The solid red line corresponds to the fit, while the dashed version is its extrapolation. The blue histogram shows the EPOS3 $D(\boldsymbol{\rho})$ distribution. The range of the horizontal axis in panels (a), (b), and (c) (in the out, side, long directions, respectively) is slightly larger than the fitted region, while in panels (d), (e), and (f) the entire investigated region (up to a meter distance) is shown (in the out, side, long directions, respectively). The kinematical selection criteria, the fit parameters, their uncertainties, as well as the fit quality is indicated on the plots as well.
• Figure 3: Lévy index $\alpha$ from EPOS3 (red line with band), compared to PHENIX data of Ref. PHENIX:2024vjp (black points), in six centrality ranges, all as a function of $m_T$. The red band shows the systematical uncertainties of the EPOS3 calculations. The statistical and systematical uncertainties of the PHENIX data points are shown as error bars and boxes, respectively.
• Figure 4: Lévy index $\alpha$ from EPOS3 (red line with band), compared to PHENIX data of Ref. PHENIX:2024vjp (black points), as a function of the number of participants $N_{\rm part}$.
• Figure 5: Lévy-scale parameters $R_{\rm out,side,long}$ from EPOS3 (red, blue, and green lines with bands, respectively), compared to PHENIX data of Ref. PHENIX:2024vjp (black points), in six centrality ranges, all as a function of $m_T$. The red, blue, and green bands show the systematical uncertainties of the EPOS3 calculations. The statistical and systematical uncertainties of the PHENIX data points are shown as error bars and boxes, respectively.